For zany conspiracy, you'd be hard-pressed IMHO to do better than The Secret Series by Pseudononymous Bosch. Gets some flack for being a Lemony clone, tho it has it's own totally unique sense of humor and kitsch and a surprising amount of effective, relatable tween angst. Also, I'm getting some (light) Artemis Fowl vibes, Colfer's sense of humor and adventure might be in your wheelhouse. Finally Cold Cereal by Adam Rex follows a ridiculously zany world-wide conspiracy involving cereal manufactures, hyper-intelligent magic children, King Arthur, and Ringo Starr. It gets more fantasy-related as the series continues, but is my gold star for readable MG featuring a truly bonkers secret conspiracy.
Oooh. Thank you for reminding me of this series. I remember reading it years ago. now i want to read it again!